The present invention relates to a coherent demodulation system for multichannel modulation system used in digital radio communication system which uses multicarrier modulation system, for instance, OFDM (Orthogonal Frequency Division Multiplexing) as modulation/demodulation system. In particular, the present invention relates to such a system which can be used in an environment in which transfer function or attenuation depends upon each subcarrier in multipath transmission circuit.
In multicarrier modulation system, such as OFDM modulation/demodulation system, a plurality of subcarriers having orthogonal relations to each other are used for transmission of communication. In a transmit side, an information signal is modulated through DQPSK (Differential Quadrature Phase Shift Keying) or 16 QAM (Quadrature Amplitude Modulation) for each subcarriers. The modulated output is applied to IFFT (Inverse Fast Fourier Transform) for inverse transformation so that block demodulation of OFDM signal is obtained.
When a DQPSK system is used for modulation of subcarriers, a delay detection system may be used for demodulation in a receive side. However, a delay detection has disadvantage that the requested Eb/No ratio must be higher by 3 dB as compared with a coherent detection system. Therefore, a coherent detection system is preferable for providing a high quality transmission circuit.
When a coherent detection system is used in a demodulation circuit in a receive side, a transmit side may use a multi-level QAM system as a modulation system. The use of a multi-level QAM system increases transmission rate of information.
When multi-paths are generated in a radio transmission channel between a transmit side and a receive side, transfer function or attenuation for propagation in the circuit depends upon each subcarriers. Therefore, if a coherent detection system is used, a channel transfer function or attenuation must be estimated for estimating amplitude information and phase information of each subcarriers which are subject to vary in each multipath circuit. Further, in a radio LAN (Local Area Network) which transfers information by using a packet, a channel transfer function or attenuation must be estimated for each packet.
A packet has in general a preamble signal at the head of each packet for synchronization purpose. Therefore, a coherent detection circuit may initially carry out channel estimation by using the preamble signal. However, if it takes long time for initial channel estimation, a throughput in a circuit would decrease, and therefore, a high speed initial channel estimation is desired. Thus, an accurate channel estimation by using short preamble signal is essential.
In a radio LAN environment, it is possible to handle that a transmission channel is quasi-static in which a transmission channel does not change during a period of each packet. In such an environment, a coherent detection is possible only by initial channel estimation.
On the other hand, when there is large variation in an environment, a coherent detection system must carry out to estimate channel transfer function or attenuation continuously.
FIG. 16 shows a block diagram of a prior OFDM coherent detection system, in which a coherent detection is carried out for each packet by using a preamble signal. This is shown in S. K. Wilson, E. K. Lhayata and J. M. Cioffi, xe2x80x9c16 QAM Modulation with Orthogonal Frequency Devision Multiplexing in a Rayleigh-Fading Environmentxe2x80x9d, Proc. of VTC""94, pp 1660-1664.
In FIG. 16, a symbol timing detection circuit 1 detects a symbol timing of a receive signal a101. An output a102 of the symbol timing detection circuit 1 is applied to a S/P (serial to parallel) conversion circuit 2.
A timing signal a114 detected by the symbol timing detection circuit 1 is applied to a control circuit 11, which generates a signal a105 for switching a preamble part and a data part of a receive signal, and for deciding the duration for holding a channel estimation signal depending upon a period of each receive packet. The signal a105 is applied to a hold circuit 8.
A Fourier transform circuit (FFT) 3 carries out Fourier transformation of an OFDM demodulation signal a103. That is, a parallel input signal a103 in time-domain is converted into a parallel signal a104 in frequency-domain. An output a104 of the Fourier transform circuit 3 is a receive vector signal of each subcarriers.
The signal a104 is applied to a P/S (parallel to serial) conversion circuit 4 which converts a parallel input signal to a serial output signal a106. The switching circuit 5 switches the serial signal a106 into a preamble signal a107 and a data signal a111 based upon the control signal a105 supplied by the control circuit 11.
The preamble signal a107 is applied to a channel estimation circuit 7, which further receives a reference preamble signal a108 from a preamble signal storage circuit 6 which stores a reference preamble signal. The channel estimation circuit 7 carries out the channel estimation for a preamble signal of a receive vector a107 of each subcarriers by using a reference preamble signal a108 according to the following equation (1).
xe2x80x83rreceive/rstorexe2x80x83xe2x80x83(1)
where;
rreceive is a received preamble signal a107 (complex number)
rstore is a reference preamble signal a108 (complex number)
The signal a109 which shows the result of the channel estimation is applied to the hold circuit 8 which holds the signal a109 for a packet period according to the control signal a105.
On the other hand, the data signal a111 is applied to a divider circuit 9 which further receives an output a110 of the hold circuit 8. The divider circuit 9 carries out the division (a111/a110) so that coherent detection is obtained. The signal a112 obtained by the coherent detection is applied to a decision circuit 10 which decides a data (0 or 1) and outputs the decided output signal a113.
As described above, a coherent detection circuit in FIG. 16 carries out the complex division of a received preamble signal by a reference preamble signal stored in the storage circuit 6 for the channel estimation.
FIG. 17 shows another prior coherent detection circuit which has a carrier filter. This is described in P. Hoeher, xe2x80x9cTCM on Frequency-Selective Land-Mobile Fading Channelsxe2x80x9d, Proc. of 5th Tirrenia International Workshop Digital Communication, Tirrenia, Italy, September 1991.
The most portions of FIG. 17 are the same as those of FIG. 16, except that FIG. 17 has a carrier filter 1011 at the output of the channel estimation circuit 107.
In FIG. 17, a symbol timing detection circuit 101 detects a symbol timing in a receive signal a1101. An output a1102 of the symbol timing detection circuit 101 is applied to a S/P conversion circuit 102.
A timing signal a1115 provided by the symbol timing detection circuit 101 is applied to a control circuit 1012, which generates a control signal a1105 according to a timing signal a1115. The control signal a1105 functions to switch a preamble part and a data part of a receive signal, and to hold a signal according to duration of a receive packet.
A parallel signal a1103 of an output of the S/P conversion circuit 102 is applied to a Fourier Transform circuit 103 which provides a receive vector signal a1104 for each subcarriers.
A P/S conversion circuit 104 converts a receive vector signal a1104 for each subcarriers into a serial signal a1106. A switching circuit 105 switches the receive vector in serial form a1106 into a preamble signal a1107 and a data signal a1111 according to the control signal a1105.
The preamble signal a1107 is applied to a channel estimation circuit 107, which further receives a reference preamble signal from a preamble storage circuit 106. The channel estimation circuit 107 carries out the channel estimation for a receive vector a1107 of each subcarriers by using a reference preamble signal a1108 from a preamble signal storage circuit 106 according to the equation (1). The signal a1109 obtained by the channel estimation is applied to a carrier filter 1011.
An output signal a1114 of the carrier filter 1011 is applied to a hold circuit 108, which further receives a control signal a1105, and holds an input signal a1114 for duration of data period of a receive packet.
A data signal a1111 is applied to a divider 109, which further receives an output a1110 of the hold circuit 108, and carries out the coherent detection. The signal a1112 obtained by the coherent detection is applied to a decision circuit 1010 for decision of data (0 or 1). Thus, the decided data a1113 is obtained.
The coherent circuit in FIG. 17 has the advantage that the accurate channel estimation is obtained because of the use of a carrier filter for the estimated channel transfer function or attenuation.
A coherent detection system provides high quality of radio communication system as compared with a delay detection system. Further, the use of a coherent detection system allows the use of multi-level QAM modulation signal, thus, higher rate communication system is obtained.
However, a prior art in FIGS. 16 and 17 which carry out the channel estimation using stored preamble signal has the disadvantage that the channel estimation would be degraded and no accurate coherent detection would be obtained in high thermal noise environment.
Further, the use of a carrier filter in FIG. 17 has the disadvantage that the size of a whole circuit of a coherent detection system is large, and the cost of the system is high.
It is an object, therefore, of the present invention to overcome the disadvantages and limitations of a prior coherent detection system by providing a new and improved coherent detection system for multicarrier modulation.
It is also an object of the present invention to provide a coherent detection system for multicarrier modulation, which can provide accurate channel estimation error under severe environment such as high thermal noise and/or deep multi-path fading.
It is also an object of the present invention to provide such a coherent detection system which is simple in structure and small in circuit size.
The basic idea of the present invention resides in that a channel estimation for a preamble signal is first carried out as is the case of a prior art. The channel estimation thus obtained is a provisional channel estimation in the present invention. The accuracy of the channel estimation is improved by processing each subcarriers in the provisional channel estimation by using an adaptive filter. The tap coefficients of the adaptive filter are controlled according to a characteristic value of each subcarrier in the provisional channel estimation.
The above and other objects are attained by a coherent detection system for multicarrier modulation system comprising; a Fourier transform circuit for receiving multicarrier modulation signal modulated with a plurality of subcarriers, and providing multicarrier demodulation so that receive vector signal of each subcarriers is obtained; a provisional channel estimation means which divides said receive vector signal of each subcarriers of an output of said Fourier transform circuit, by a reference transmit vector signal or a transmit vector obtained by demodulation of a receive signal, so that transfer function or attenuation of each subcarriers on propagation is estimated; a filter means for smoothing estimated channel transfer function from said channel estimation means among subcarriers; a divider means for dividing said receive vector signal by estimated channel transfer function or attenuation from said filter means so that said receive vector signal is coherent detected; wherein said filter means is an adaptive filter constituted by a transversal filter in which at least one of a number of taps and tap coefficient of each taps is adaptively controllable; a characteristic extraction circuit receiving estimated channel attenuation of each subcarriers obtained by said channel estimation means is provided; a selection circuit for selecting at least one of a number of taps and tap coefficient of each taps of said adaptive filter according to an output of said characteristic extraction circuit is provided; and said selection circuit selects one of outputs of said adaptive filter for each subcarriers according to an output of said characteristic extraction circuit so that estimated channel attenuation for each subcarriers is provided.
The channel estimation by said channel estimation means is a provisional channel estimation, which is subject to improve by using an adaptive filter. Said divider means provides coherent detection by dividing a receive vector by an improved channel estimation of each subcarrier.
It should be noted that a receive signal which is received through a multipath transmission channel having deep fading is subject to amplitude variation for each receive vector of a subcarrier. Further, thermal noise is superimposed to a receive signal. The present invention removes the affection of noise and fading by using an adaptive filter.
However, if a filter having fixed tap coefficients is used for all the estimated subcarriers, no accurate estimation is possible when amplitude variation due to fading is large. Therefore, the present invention controls tap coefficients of an adaptive filter according to a characteristic value of a subcarrier in order to suppress the degradation of channel estimation by fading.
The present invention uses a plurality of filters each having different tap coefficient from one another for smoothing estimated channel attenuation between subcarriers, therefore, excellent channel estimation is obtained by selecting one of the filter outputs adaptively. In an embodiment, a selection means selects an output of an adaptive filter according to an output signal of a characteristic extraction means for each subcarriers, and therefore, difference of amplitude and/or phase rotation among subcarriers provides less affection o channel estimation.
A plurality of tap coefficients of an adaptive filter are implemented merely by bit shift operation in any embodiment. Assuming that tap coefficient of a center tap is 1, and a tap coefficient of another tap is a power of (1/2), a product multiplied by a tap coefficient is obtained merely by bit shift of a signal according to a tap position. Therefore, a plurality of filter means are obtained with no additional multiplier, and size of a circuit may be small.
A characteristic value of said characteristic extraction means is, for instance, estimated amplitude of a subcarrier, estimated phase rotation between adjacent subcarriers, product of estimated amplitude and estimated phase rotation between adjacent subcarriers, vector difference between adjacent subcarriers, and product of estimated amplitude and vector difference.
The larger a characteristic value is, the larger the tap coefficient of an adaptive filter is.
Preferably, a symbol timing detection means is provided for receiving multicarrier modulation signal modulated on a plurality of subcarriers, and detecting a symbol timing in the received signal. Said Fourier Transform is carried out for the signal with a symbol timing detected.
Preferably, a phase compensation means is provided between said channel estimation means and said adaptive filter means, so that constant phase rotation between adjacent subcarriers is preliminary compensated, and compensated channel attenuation is applied to the adaptive filter.
Preferably, a phase-compensated extrapolation means and a phase compensation are provided between said channel estimation means and said adaptive filter means, so that said extrapolation means receives said estimated channel attenuation of subcarriers at extreme ends in pass band, effects phase compensation to a received signal according to constant phase error between subcarriers due to an error of an OFDM symbol detection window in said symbol timing detection means, and extrapolates phase compensated signal, when signals out of the pass band is requested in said adaptive filter for processing a subcarrier close to end of the pass band, and said phase compensation means receives estimated channel attenuation within the pass band and extrapolated signals out of the pass band provided by said phase-compensated extrapolation means, effects compensation of constant phase rotation in received signals, and applies phase compensated signals to said adaptive filter means.
Preferably, a characteristic value compensation means (1217) is provided between said channel estimation means (208) and said characteristic extraction means (A), so that said characteristic value compensation means (1217) receives an output of said channel estimation means (208) including constant phase error due to OFDM symbol detection window error in said symbol timing detection means, compensating said constant phase error and applying phase compensated signal to said characteristic extraction means (A).
Preferably, an extrapolation means is provided between said channel estimation means and said adaptive filter, so that said extrapolation means receives said estimated channel attenuation of subcarriers, and extrapolates said estimated channel attenuation of subcarriers at extreme ends in pass band, when signals out of the pass band is requested in said adaptive filter for processing a subcarrier close to end of the pass band.
Preferably, said adaptive filter means is controlled by a measured signal of modulation system of each subcarrier.
Preferably, said adaptive filter means is a transversal filter which controls bandwidth of the filter by a number of taps and tap coefficient of each taps of the filter.